Guide · Financial Calculator
How to calculate your student loan payment
Student loans are a classic Time Value of Money problem: you borrow a lump sum today and repay it with interest over a fixed period. Understanding the numbers behind your loan — not just the monthly payment, but the total interest you'll pay — puts you in a stronger position to manage or pay it down faster.
What each TVM field means for student loans
Switch Pebble to Financial mode (tap the mode button and select 💰). Here's how to map your loan details to the five fields:
- N — Repayment term in months (e.g. 10 years = 120)
- I/Y — Annual interest rate (e.g. 6.5 for 6.5%)
- PV — Total loan balance you're repaying
- PMT — Monthly payment (what you want to find)
- FV — Enter 0 (the loan is fully paid off at the end)
Example: 10-year standard repayment
You graduate with $35,000 in federal student loans at a 6.5% annual interest rate. You enrol in the standard 10-year repayment plan. What will your monthly payment be, and how much interest will you pay in total?
Enter these values, then tap CPT → PMT
Over 10 years you'll pay roughly $47,760 in total — about $12,760 in interest on top of the $35,000 principal. That's a manageable amount compared to longer repayment terms.
Example: 20-year extended repayment
Same $35,000 loan at 6.5%, but stretched over 20 years to lower the monthly burden. How much does extending the term actually cost you?
Enter these values, then tap CPT → PMT
The monthly payment drops by $137, but the total interest rises to around $27,640 — more than double the 10-year figure. The extra decade of repayments costs you an additional $14,880 in interest alone.
Should you make extra payments?
You can use the financial calculator in reverse to model accelerated repayment. Say you can afford $550 per month instead of $398 — enter that as PMT (negative), keep PV at $35,000 and I/Y at 6.5, set FV to 0, then tap CPT → N. The result tells you exactly how many months — and how many years of interest — you'll shave off your loan.